The following relates to the oncological arts, medical imaging arts, positron emission tomography (PET) arts, and related arts.
Radiation therapy has been shown to be effective in treating certain types of cancer. In these techniques, a high dosage of ionizing radiation is applied to the tumor or other malignant tissue. The typically high tissue growth rate and “unnatural” nature of the malignant tissue results in the ionizing radiation preferentially damaging or killing the malignant tissue; however, healthy tissue is also adversely impacted by the ionizing radiation. Accordingly, the ionizing radiation is applied in a tomographic or multi-beam configuration with the beam intensities and, optionally, cross-sectional beam profiles designed to deliver a therapeutic radiation dosage to the target tumor while keeping the radiation dosage received by neighboring critical organs below a design threshold.
Because anatomy varies amongst individuals, radiation therapy is planned on an individual basis for each patient. Anatomical data for the radiation therapy planning is acquired using computed tomography (CT) imaging which measures transmission of a tomographically rotating x-ray beam to enable reconstruction of a three-dimensional image of the internal patient anatomy. CT imaging has numerous advantages for this application—data acquisition is fast (of order a few minutes); it typically provides detailed structural information about both the target tumor and the surrounding critical organs; the x-rays mimic the transmission and absorption characteristics of the therapeutic radiation such that a therapeutic radiation absorption map can be constructed from the CT image using suitable tissue absorption correction factors; and CT scanners can be constructed with large bores.
A typical radiation therapy system has a patient aperture of order 80 centimeters. For example, some commercially available linear accelerator (linac)-based radiation therapy systems have patient apertures of 85 centimeters. Problematically, internal organ positions can shift, rotate, compress, expand, or otherwise differ depending upon the precise patient position. As a result, if the patient has to assume a different position in the CT scanner than in the radiation therapy system due to a smaller patient aperture of the former, then the anatomical images used for therapy planning may not accurately replicate the anatomical configuration during therapy.
By providing a CT scanner with a bore at least as large as that of the radiation therapy system, the patient can be positioned in the CT scanner in the same way the patient is positioned in the radiation therapy system. For example, the patient can have his or her arm or arms elevated or otherwise positioned in such a way as to not impede the therapeutic radiation beam; and, the patient can be positioned in precisely this same way in the large-bore CT scanner. This helps to ensure that organ positions are the same in both the planning CT images and in the radiation therapy system. One CT scanner that has a large bore suitable for radiation therapy planning is the Brilliance™ CT Big Bore™ system (available from Koninklijke Philips Electronics N.V., Eindhoven, the Netherlands), which has a patient aperture of 85 cm.
CT has been, and is expected to remain, a primary tool for acquiring patient anatomical data for planning. However, other imaging modalities could also be useful for radiation therapy planning.
Positron emission tomography (PET) imaging is a good candidate as a secondary imaging modality, because PET provides functional information that is complementary to the structural information provided by CT. In PET imaging, a radiopharmaceutical that emits positrons is administered to the patient. Each emitted positron interacts with a nearby electron in an electron-positron annihilation event that emits two oppositely directed 511 keV gamma rays. These gamma rays are detected substantially simultaneously by a radiation detector ring, thus defining a line-of-response along which the electron-positron annihilation event must (neglecting scattering) have occurred. In “time-of-flight” PET, the slight time difference (or lack thereof) between the two substantially simultaneous gamma ray detection events is used to further localize the annihilation event along the line of response.
The information provided by PET depends upon the radiopharmaceutical and its interaction with the patient physiology. A radiopharmaceutical injected into the bloodstream can inform about blood flow or blood perfusion. Since cancerous tumors tend to have dense vasculature, they are strongly visible in PET images that employ a blood-borne radiopharmaceutical. Similarly, radiopharmaceuticals including glucose cluster in areas of high metabolic activity, such as a cancerous tumor. On the other hand, if the tumor tissue is necrotic (perhaps due to success of previous radiation therapy sessions) then the tumor may be visible in CT images (since it remains structurally intact) but not in PET images (since it is necrotic and is no longer being actively fed by blood flow in the vasculature or metabolizing glucose).
A substantial obstacle to utilizing PET in radiation therapy planning and therapy monitoring is that PET scanners do not have the requisite combination of large patient aperture and high sensitivity. The radiopharmaceutical dosage is strictly limited due to safety concerns. As a result, a PET imaging session can take of order 30 minutes or longer to acquire sufficient data for reconstructing an image useful for oncology. The imaged volume increases with the square of the scanner diameter, which complicates scaling up the geometry to larger size (for example, to an 85 cm patient aperture). Stabilization of the patient in the PET scanner is another issue, especially in some hybrid CT/PET scanners in which the patient is supported by cantilevering in the PET gantry.
A blood borne radiopharmaceutical accumulates in areas of high blood concentration. These areas include the cancerous tumor due to its dense vasculature, but also include the brain and the bladder (the latter being the organ by which the radiopharmaceutical is ultimately excised from the body). The brain and bladder are therefore substantial sources of stray radiation that can lead to false gamma ray counts that degrade image quality. Use of energy-range and time-range filtering reduces these stray counts; however, in human-sized PET scanners these post-acquisition filtering techniques have been found to be insufficient, by themselves, to obtain clinical quality images for oncological applications.
With reference to FIG. 1, therefore, in existing human-sized PET scanners radiation shield rings are included to physically block this stray radiation from reaching the radiation detector ring. FIG. 1 diagrammatically depicts the detector ring 10 in cross-section so as to show the radiation detectors comprising (in this embodiment) a scintillator 12 viewed by photomultiplier tube (PMT) detectors 14. A patient S is arranged with a tumor or other malignancy of interest positioned at an isocenter 16 of the detector ring 10. Side shield rings 20 made of lead or another material with high radiation stopping power extend radially inwardly from the ring of PET detectors so as to block a substantial portion of stray radiation from the brain region B and bladder or kidney region K from reaching the scintillator ring 12. To further illustrate with some typical dimensions, in one commercial PET scanner (the Gemini™ Time-of-Flight PET/CT scanner, available from Koninklijke Philips Electronics N.V., Eindhoven, the Netherlands), the scintillator ring 12 has a diameter of 89 cm, and the shield rings 20 extend radially about 10 centimeters inward, so that the shield rings 20 have an inner diameter of 70 cm. As a result, an exterior housing 22 can have a patient aperture of no larger than about 70 cm, that is, no more than about 79% of the detector ring diameter. A patient aperture of 70 cm is large enough to receive a human subject, but is too small for performing imaging of a human subject in the same position as the subject would assume in the radiation therapy system. For example, the patient could not assume the conventional “frogleg” position used in radiation therapy of the colorectal region.
It has long been believed in the PET imaging arts that side shielding (such as the side shield rings 20) are essential for obtaining oncological quality images. (As used herein, the term “oncological quality images” and the like encompasses images of high resolution and low noise that are suitable for medically acceptable analysis of cancerous tumors and other malignancies for patient health-critical purposes such as radiation therapy planning or other interventional therapy planning or planning decision-making.) For example, Mullani, U.S. Pat. No. 4,559,597 issued in 1985 discloses time-of-flight PET and considers the possibility of operating without side shields (called “septa” in Mullani), but concludes that omitting side shields introduces a substantial amount of scatter in the data and may not be acceptable for a clinically usable system. Mullani concludes that septa of length 12 cm provides optimal performance. Commercial oncology-quality PET scanners today continue to use side shielding of order 10 cm.
Other issues with oncological PET relate to scan time and patient inconvenience. In contrast to CT acquisition which is of order a few minutes, an oncological-quality PET scan can take 30 minutes or longer. Both faster acquisition and improved fusion of CT and PET images could be achieved by using a hybrid PET/CT scanner, in which the CT and PET gantries are arranged coaxially together to receive a subject via a common subject support system. Hybrid PET/CT scanners are available, as exemplified by the aforementioned Gemini™ PET/CT scanner. In most cases, CT is the “primary” imaging modality used in these systems, and hence is positioned closest to the patient loading end. When PET imaging is to be performed, the subject is transferred through the CT gantry and thence into the PET gantry.
However, this arrangement results in cantilevered support of the patient in the PET scanner, with the possible consequence of downward deflection of the patient support and resultant PET/CT image misalignment. Such misalignment can be corrected mathematically during the post-acquisition PET/CT image fusion processing. However, the correction increases computational complexity, and introduces yet another potential source of error in fusing CT and PET images for radiation therapy planning.
The cumulative effect of these issues has led to continued dominance of CT as the radiation therapy planning modality of choice, with PET imaging relegated to an infrequently used secondary planning modality. For example, a recent survey indicates that close to 90% of radiation therapy planning acquisitions employed CT imaging, while less than 10% of those acquisitions also employed PET imaging as a secondary modality. See IMV 2006 Radiation Oncology Census Market Summary Report, March 2007.
The following provides a new and improved apparatuses and methods which overcome the above-referenced problems and others.